1. Field of the Invention
The present invention relates to a method and apparatus for linear correction of the non-linearity of a digital data output characteristic for an analog signal input to an analog-to-digital (A/D) converter.
2. Description of the Related Art
Known in the past has been an A/D converter provided with a pulse delay circuit comprised of a plurality of inversion circuits connected in a ring and applying the input voltage to be converted to digital data as the power supply voltage of the inversion circuits for A/D conversion of the input voltage utilizing the difference in the delay times of the inversion circuits arising from the power supply voltage (for example, see Japanese Unexamined Patent Publication (Kokai) No. 5-259907).
This type of A/D converter (hereinafter referred to as a “time A/D converter” or “TAD”) is comprised of a pulse delay circuit for sending round a start pulse SP, a latch and encoder for detecting (latching) an arrival position of the start pulse in the pulse delay circuit at a rising (or falling) timing of a sampling clock input from the outside and outputting the detection result converted to predetermined bits of digital data expressing what number stage the delay unit through which the start pulse passed is from the head, a counter for counting the number of cycles of the start pulse in the pulse delay circuit, and a latch circuit for latching the count of the counter at the rising (or falling) timing of the sampling clock.
The output characteristic of such a TAD usually has non-linearity. For example, it has an output characteristic bulging upward from the straight line of the desired output characteristic (ideal A/D line).
Such a TAD having non-linearity becomes a problem for example in sensing equipment, measuring equipment, etc. where precise linearity is required and is difficult to use as an A/D converter in such sensing equipment etc. As one method for solving the problem of the non-linearity of the TAD output characteristic, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-259907 discloses technology for storing correction values corresponding to non-linearity of output data in a correction use ROM and using the correction values to correct the A/D converted output data. By using this, an A/D converted output characteristic changing linearly with respect to changes in input voltage can be obtained.
Summarizing the problem to be solved by the present invention, a TAD output has a characteristic of fluctuating even due to temperature changes. That is, the delay times of the delay units forming the pulse delay circuit change not only due to the voltage input as the power supply voltage, but also the temperature. Therefore, the non-linearity of the A/D converted output characteristic also changes due to the temperature.
To deal with this, for example, it may be considered to separately provide a temperature measurement circuit, store correction values based on temperature in the correction use ROM, and make corrections in accordance with the temperature. With this method, however, it is necessary to prepare correction values for the different temperatures in a presumed temperature range. This would invite an increase in size of the ROM and greater complexity of the circuit configuration and would become a factor behind higher costs.
Further, not only in a TAD, but in a general A/D converter as well, the A/D converted output data generally includes non-linear error. Further, the output value ends up changing due to a change in the ambient temperature, i.e., so-called “temperature drift” occurs. Therefore, even if the method of correction using a correction use ROM is applied for correction of non-linearity of a general A/D converter, the above type of problem will still end up occurring.